1. Field of the Invention
The present invention relates to a semiconductor device using crystalline semiconductor, and to a process for fabricating the same.
2. Prior Art
Thin film transistors (hereinafter referred to simply as “TFTs”) are well known as devices utilizing thin film semiconductors. The TFTs are fabricated by forming a thin film semiconductor on a substrate and processing the thin film semiconductor thereafter. The TFTs are used in various types of integrated circuits, and are particularly noted in the field of electrooptical devices; especially, they attracted much attention in the field of switching elements that are provided to each of the pixels of active matrix(addressed) liquid crystal display devices and driver elements of the peripheral circuits thereof.
Amorphous silicon film can be utilized most readily as the thin film semiconductors for TFTs. However, the electric characteristics of the amorphous silicon film are disadvantageously poor. The use of a thin film of crystalline silicon can solve the problem. Crystalline silicon films are known as polycrystalline silicon, polysilicon, microcrystalline silicon, etc. The crystalline silicon film can be prepared by first forming an amorphous silicon film, and then heat treating the resulting film for crystallization.
However, the heat treatment for the crystallization of the amorphous silicon film requires heating the film at a temperature of 600° C. or higher for 20 hours or longer. Such a heat treatment has a problem that it is difficult to use a glass as a substrate. For instance, a Corning 7059 glass commonly used for the substrate of active matrix liquid crystal display devices has a glass deformation temperature of 593° C., and is therefore not suitable for using in large area substrates that are subjected to heating at a temperature of 600° C. or higher. That is, if a commonly used Corning 7059 glass substrate is heated at a temperature of 600° C. or higher for 20 hours or longer, distinct shrinking and warping occur on the substrate.
The aforementioned problem can be overcome by performing the heat treatment at a temperature as low as possible. On the other hand, from the viewpoint of increasing productivity, it is required to shorten the duration of this heat treatment step as much as possible.
In case of crystallizing an amorphous silicon film by heating, moreover, the entire film becomes crystalline. Accordingly, it is not possible to crystallize the film locally, or to control the crystallinity of a particular region.
To overcome the aforementioned problems, JP-A-2-140915 and JP-A-2-260524 (the term “JP-A-” as referred herein signifies an “unexamined published Japanese patent application”) propose a technique which comprises artificially introducing portions or regions inside an amorphous silicon film to provide sites as the crystallization nuclei, and then heat treating the amorphous silicon film thereafter to crystallize the film selectively. The technology allows crystal nuclei to form at predetermined sites within the amorphous silicon film.
According to the constitution of JP-A-2-140915, for instance, an aluminum layer is formed on the amorphous silicon film, and crystal nuclei are allowed to generate in the portion at which the amorphous silicon is in contact with aluminum. Thus, by heat treating the resulting film, crystal growth is initiated from the thus provided crystal nuclei. JP-A-2-260524 proposes a constitution which comprises adding tin (Sn) into an amorphous silicon film by means of ion implantation, and then generating crystal nuclei from the region into which tin ions are added.
Aluminum (Al) and tin (Sn) are substitutive metallic elements. Thus, they cannot diffuse and intrude deeply into the silicon film because they readily form an alloy with silicon. Accordingly, the alloy serves as the crystal nuclei, and the crystallization in this case proceeds from these alloy portions. The crystallization process using Al or Sn is characterized in that the crystal growth occurs from the portion into which Al or Sn is introduced (i.e., the alloy layer of Si and Al or Sn). In general, crystallization proceeds in two steps; a first step of generating initial nuclei, and a subsequent step of crystal growth which occurs from the initial nucleation sites. The metallic elements of the substitutive type, i.e., Al and Sn, are effective for generating the initial nucleation sites, but have almost no effect on the later step of crystal growth.
Thus, the temperature of crystallization cannot be lowered nor the duration of crystallization be shortened by using Al or Sn. That is, the method using Al or Sn is none the better as compared with the conventional crystallization of simply heating the amorphous silicon film.
According to the study of the present inventors, it is possible to crystallize an amorphous silicon film by heating the film at 550° C. for about 4 hours. This can be accomplished by first depositing a trace amount of an intrusion type element, such as nickel or palladium, on the surface of the amorphous silicon film, and heating the resulting product thereafter. The intrusion type metallic elements not only facilitates the initial nucleation, but also accelerates the later crystal growth. Thus, as compared with a conventional process which comprises simply heating the film, the heating temperature can be lowered, and the duration of heating can be shortened.
The elements above (i.e., catalyst elements which accelerate the crystallization) can be introduced into the amorphous silicon film in a trace quantity by means of plasma treatment, vapor deposition, or ion implantation. Plasma treatment as referred herein signifies adding the catalyst elements into the amorphous silicon film by effecting the treatment in a plasma CVD apparatus of a parallel plate type or a positive column type by using a material containing the catalyst element as the electrode, and allowing a plasma to generate under, for instance, gaseous nitrogen or gaseous hydrogen.
Metallic elements which accelerate the crystallization above are the intrusive elements such as Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, Ag, and Au. These intrusive elements diffuse into the silicon film during the heat treatment step. The crystallization proceeds with progressive diffusion of these intrusive elements. That is, the intrusive elements accelerate the crystallization of the amorphous silicon film by exerting catalytic function at every site they proceed inside the amorphous silicon film.
Thus, by incorporating the intrusive elements to the amorphous silicon film, crystallization can be accelerated in a manner differing from that which proceeds gradually from the crystal nuclei. For instance, if an intrusive element is introduced into a particular portion of an amorphous silicon film and is subjected to heat treatment thereafter, crystallization proceeds from the site the metallic element was introduced, and in a direction parallel to the surface of the film. The length of the thus crystallized region amounts to several tens of micrometers or even longer. Furthermore, by introducing the metallic element above to the entire surface of the film, the film can be wholly and yet uniformly crystallized. As a matter of course, the entire film exhibits a polycrystalline or a microcrystalline structure, but the grain boundary thereof is not distinguished. Accordingly, a device having stability in characteristics can be obtained by using the desired portion of the film.
The intrusive elements above rapidly diffuse into the silicon film. Accordingly, the key of this method is the estimation of the quantity to be introduced (added). If the elements are introduced insufficiently, the addition of the elements results in a small effect. A film with favorable crystallinity cannot be expected in such a case. If the elements are introduced in an excessive quantity, on the other hand, the semiconductive characteristics of silicon would be impaired.
Accordingly, the optimum quantity of the intrusive metallic elements above must be estimated. For instance, Ni effectively accelerates the crystallization if it is added into the crystallized silicon film at a concentration of 1×1015 cm−3 or higher. So long as the concentration of nickel does not exceed 1×1015 cm−3, the semiconductive characteristics of the silicon film remains without being impaired. The concentration of the elements in this case is defined as the minimum value obtainable by SIMS (secondary ion mass spectroscopy).
The above description applies not only to nickel, but also to the metallic elements other than nickel enumerated above. Thus, the same effect is expected on the other metallic elements so long as they are added at a concentration in a range defined above.
To control the concentration of the metallic elements above (those metallic elements capable of accelerating crystallization are referred to hereinafter as “catalyst elements”) inclusive of nickel in an optimum range for accelerating crystallization, the quantity thereof must be controlled at the point of their introduction.
Considering a case of using nickel as the catalyst element, an amorphous silicon film was deposited, and nickel was added therein by plasma treatment to fabricate a crystalline silicon film. The progress of crystallization and the like was studied in detail. The following points were found as a result:    (1) In case nickel is introduced into the amorphous silicon film by means of plasma treatment, nickel has already been intruded into the amorphous silicon film to a considerable depth before subjecting the film to heat treatment;    (2) Initial crystal nucleation occurs from the surface at which nickel was introduced; and    (3) In case of depositing nickel on an amorphous silicon film by evaporation, crystallization occurs in a manner similar to that occurred in plasma treatment.
It can be found from the above findings that not all nickel introduced by plasma treatment function sufficiently effective. That is, even if nickel is introduced in a large quantity, it does not follow that all the nickel atoms function sufficiently. It is therefore assumed that the point (or plane) at which nickel contacts silicon functions to decrease the temperature of crystallization. Conclusively, nickel atoms are preferably dispersed as finely as possible in the amorphous silicon film. In other words, “nickel atoms need to be introduced in the vicinity of the surface of amorphous silicon film at a minimum concentration necessary for realizing the low temperature crystallization of the amorphous silicon film”.
Vapor deposition can be mentioned as a method for introducing nickel atoms in a trace amount in only the vicinity of the surface of the amorphous silicon film, i.e., introducing catalyst elements capable of accelerating the crystallization in a trace amount in only the vicinity of the surface of the amorphous silicon film. However, vapor deposition is inferior concerning its controllability, and it fails to precisely control the quantity of catalyst element to be introduced in the amorphous silicon film.
Furthermore, it is required to minimize the quantity of the catalyst element to a level as low as possible. However, there is still a problem that the crystallinity becomes an impurity.